I don't believe there is a maximum file size for assignment submissions, but certain applications (such as Turnitin) may have their own. However elsewhere I think there's a 50MB upload limit on things like file attachments. A common workaround is to provide a link to the file stored elsewhere (such as Dropbox or Google Drive). Not really sure what the last question is asking...

Students can use outside applications to submit an assignment - such as OneDrive or Google Drive or Dropbox. However, those files are submitted to Speedgrader just like a word doc uploaded to Canvas. In the case where the student has submitted something outside of the traditional submission button (such as an email) the instructor can't submit the file on the student's behalf, but can use the Speedgrader to provide feedback to the student and attach any relevant documents (such as a marked up word document).

To expand a bit on what Anthony Bunag has already mentioned... the outside applications (3rd party) are called LTI integrations. Each LTI integration works differently, and some do allow for file submissions (as Anthony has mentioned above). If you would like to learn more about LTIs, you can visit this portion of the Guides:

Also, if you have O365 or Google integrated in Canvas with their LTIs, here are a couple Guides for you that detail how to get things set up so that your students can submit work to you from those services: